Reacción a "Evidence on the toxicity of microplastics in the human brain and strategies for their elimination examined"

The commentary Human microplastic removal: what does the evidence tell us?, published in Brain Medicine, refers to a recent article in Nature Medicine (Brief Communication) and places it in the context of other worrying information on the possible impact of plastic pollution on human health. I refer firstly to the Nature Medicine article, Bioaccumulation of microplastics in decedent human brains, which reports very high concentrations of plastics in autopsied brains and suggests a possible link to the development of dementia. In my opinion, these findings are flawed for the following reasons:

1. Inadequate methodology. The study uses pyrolysis coupled to gas chromatography with mass spectrometry detection (Py-GC-MS), a technique unsuitable for the analysis of biological matrices due to interference with non-specific pyrolysis products. This has been pointed out recently, although it was already a known fact in specialised media.
2. Unlikely concentrations. Concentrations of plastic in brains are reported with median values of 3.3 to 4.9 mg/g, extraordinarily high values, 50 times higher than those found in sewage sludge. Furthermore, the concentrations in brain are higher than those detected in liver and kidneys, organs presumably more exposed and lacking the protection of the blood-brain barrier, which does not allow sizes larger than 20-50 nm to pass through. It is not possible to make assertions of this calibre without providing evidence on the mechanisms that would explain such a concentration and, especially, why it would occur precisely in the brain. The most reasonable explanation is interference with lipid matter in brain tissue.
3. Failure to identify particles as plastics. Contrary to what is mentioned in the Brain Medicine commentary, the original study does not identify any particles as plastic. The 100-200 nm particles observed in light and electron microscopy images are not identified as such; it is merely suggested that they might be plastics. However, neither optical nor electron microscopy (TEM or SEM) are suitable techniques for the identification of microplastics or nanoplastics.
4. Problems in sample collection and quality control. Autopsy samples were obtained under non-detailed conditions, with insufficient contamination and quality control measures. This is especially critical considering that the hospital environment is particularly rich in plastics, which implies a high risk of contamination. This factor alone should have been sufficient reason to reject the article.

Subsequently, the commentary reflects on the possible impact of plastic pollution, a widely known issue, on which I can make some qualifications:

1. Accumulation of plastics in the human body. It is true that plastic waste pollution has increased in recent decades due to the irrational use of this material and its inadequate waste management. However, this alone is not proof that we are accumulating plastic in our bodies.
2. Toxic effects in the laboratory. It is also true that laboratory studies show that exposure to plastics, both the polymer and the many additives that accompany it, can have toxic effects on a variety of organisms. However, these tests are almost always conducted at concentrations much higher than those found in natural environments or the exposure concentrations to which we may be subjected through food, which limits the extrapolation of these results to real-life scenarios.
3. Real exposure to plastic. It is true that we are exposed to plastics, but the data presented in terms of numbers of particles per unit volume are misleading. Our own calculations, obtained on samples of water bottled in plastic (PET), indicate that the average concentration is 1.61 µg/L (Scientific Reports). This implies an exposure of 4-18 ng per kilogram of body weight per day or, in other words, a person consuming 2 litres of water per day would need 850 years to ingest 1 g of plastic.

In short, both the commentary and the communication to which it refers are alarmist, based on erroneous or biased data, and suggest a link between plastic and pathologies in humans that cannot be affirmed with the knowledge currently available.